Various display units of an advertising light, a marker light, an evacuation light and so forth are set up in places of offices, shops, roads, transportation and so forth. Some of the display units are visible from both sides thereof. In the most popular display unit of so-called light box, light sources of fluorescent lamps and so forth are disposed inside a rectangular parallelepiped comprising a transparent plate. Letters and designs drawn on the transparent plate are illuminated by the light source from the inside to perform double-side display.
In the case of the light box, if a distance between the light source and the transparent plate is merely shortened for the purpose of downsizing, irradiation unevenness occurs and it becomes difficult to recognize the letters and the designs. In consideration of this, a light diffusion sheet is attached to the back of the transparent plate to reduce the irradiation unevenness. In virtue of this, it is possible to thin a width from 100 mm or more to about 60 mm. However, there arises a problem in that brightness is lowered due to loss caused by light absorption and reflection of the light diffusion sheet.
In order to solve the above problem, an illumination-light reflective display device is proposed (see Japanese Patent Publication No. 06-100886). In this display device, two thin transparent light guide plates, both surfaces of which are coated with light-guiding transparent resin and one surface of which is roughened and has letters and designs drawn thereon, are attached to each other so as to make the light of the light source enter from the periphery of the attached light guide plates.
However, as to the display device described in the above-noted Publication, the utilized light of the light source is only the light applied to the periphery of the light guide plates and a part of the reflected light. Since the light is inefficiently utilized, it is necessary to brighten the light source for the purpose of obtaining the desired brightness. This causes a problem in that electric power consumption increases. Further, since the brightness of the light source has a limitation, there arises a problem in that a size of the light guide plate is restricted, and consequently, a display size is restricted.